Various forms of contact units have been used in contact probes and probe cards for testing printed circuit boards, semiconductor devices, and semiconductor wafers, as well as in electric sockets and connectors for semiconductor devices and others. For instance, in testing a semiconductor wafer, a DC test is initially carried out, and after cutting the wafer into small chips, an AC test is carried out on each of the chips.
According to a conventional wafer test, as illustrated in FIG. 16, a test unit, which is normally called a probe card and comprises a planar support member 32 and needle-shaped electroconductive contact members 31 mounted on the support member 32, is applied to a wafer 2 which is desired to be tested as an object to be accessed. The contact members 31 are defined by forming electroconductive and resilient wire members into the shape of a rake. According to this arrangement, because the needle section of each of the contact members 31 is relatively long, only a relatively poor electric property can be achieved, and the contact unit is normally inadequate for handing signals faster than 50 MHz. Also, if the objects to be contacted are spherical in shape, the contact may become unstable because of the tendency of the contact members to slip over the surface of the parts to be contacted.
To meet the demand for an accessing capability for high speed signals, membrane type contact units have been developed, and a membrane probe as illustrated in FIG. 17 is known as such a membrane type electroconductive contact unit. In this membrane probe, a plurality of bumps 35 serving as contact members are formed on a film base 34 which is in turn mounted on a support member 33. However, because the bumps 35 are not resilient by themselves, a stable contact pressure (electric contact resistance) cannot be attained.
Therefore, as illustrated in FIG. 17, a high pressure air is applied to the side of the film base 34 facing away from the bumps 35 to establish a required state of contact between the bumps 35 and the wafer 2. However, the heights of the bumps 35 are not necessarily uniform, and the wafer surface may not be completely planar so that the system may be unable to accommodate such irregularities in the heights of the bumps, and the electric contact resistance may therefore become unstable.
According to a known electroconductive contact unit suitable for use in contact probe units for testing electroconductive patterns on printed circuit boards and semiconductor devices, an electroconductive needle member is received in each tubular holder so as to be axially moveable into and out of the tubular holder, and is resiliently urged out of the tubular holder until it is restricted from moving further out of the tubular member. According to such an electroconductive contact unit, the forward end of the electroconductive needle member is resiliently pushed against an object to be tested so that an electric signal may be transmitted between the object to be tested and an external circuit.
However, when an electric current flows through the compression coil spring, it produces an inductance which is proportional to the square of the number of turns. Therefore, when the electric signal flowing through the contact probe consists of a high frequency signal (in the order of tens of MHz to several GHz), the high frequency signal flows though the coiled electroconductive member, and the resulting increase in the inductance and electric resistance may adversely affect the electric properties of the detected signal.